Photodesorption in carbon nanotubes

ABSTRACT

Carbon nanotube devices are manipulated in a manner that is useful for a variety of implementations. According to an example embodiment of the present invention, light ( 632 ) is used to photodesorb molecules from a carbon nanotube ( 620 ).

RELATED PATENT DOCUMENTS

This patent application is the national stage filing under 35 U.S.C. §371 of International Application No. PCT/US2002/012033 filed on Apr. 18,2002; which claims benefit under 35 U.S.C. § 119(e) of U.S. ProvisionalApplication Ser. No. 60/284,846 filed on Apr. 18, 2001, both of whichare incorporated herein by reference.

This patent document also relates to application Ser. No. 09/574,393(now U.S. Pat. No. 6,528,020), filed on May 19, 2000 and entitled“Carbon Nanotube Devices,” which is a divisional/continuation-in-part ofapplication Ser. No. 09/133,948 (now U.S. Pat. No. 6,346,189), filed onAug. 14, 1998 and entitled “Carbon Nanotube Structures Made UsingCatalyst Islands,” and which claims priority to U.S. ProvisionalApplication Ser. No. 60/171,200, filed on Dec. 15, 1999; and to U.S.Provisional Patent Application Ser. No. 60/335,306, entitled “IntegratedNanotubes for Electronic Noses” and filed on Nov. 1, 2001, all of whichare fully incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was supported in part by grant number ECS 9871947 fromthe National Science Foundation (NSF). The U.S. Government has certainrights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to carbon nanotubes and moreparticularly to photo-induced desorption in carbon nanotubes and relatedapplications.

BACKGROUND

Carbon nanotubes are unique carbon-based, molecular structures thatexhibit interesting and useful electrical properties. There are twogeneral types of carbon nanotubes, referred to as multi-walled carbonnanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs). SWNTs havea cylindrical sheet-like, one-atom-thick shell of hexagonally-arrangedcarbon atoms, and MWNTs are typically composed of multiple coaxialcylinders of ever-increasing diameter about a common axis. Thus, SWNTscan be considered to be the structure underlying MWNTs and also carbonnanotube ropes, which are uniquely-arranged arrays of SWNTs.

Due to their unique electrical properties, carbon nanotubes are beingstudied for development in a variety of applications. These applicationsinclude, among others, chemical and bio-type sensing, field-emissionsources, selective-molecule grabbing, nano-electronic devices, and avariety of composite materials with enhanced mechanical andelectromechanical properties. More specifically, for example, inconnection with chemical and biological detection, carbon nanotubes arebeing studied for applications including medical devices, environmentalmonitoring, medical/clinical diagnosis and biotechnology for genemapping and drug discovery. For general information regarding carbonnanotubes, and for specific information regarding SWNTs and itsapplications, reference may be made generally to the above-mentionedpatent documents, and also to: “Carbon Nanotubes: Synthesis, Structure,Properties and Applications,” M. S. Dresselhaus, G. Dresselhaus and Ph.Avouris (Eds.), Springer-Verlag Berlin Heidelberg, New York, 2001; and“T. Single-shell Carbon Nanotubes of 1-nm Diameter,” Iijima, S. &Ichihashi, Nature 363, 603-605 (1993).

In these and other carbon nanotube implementations, nanotube devicesexhibiting both high functionality and high flexibility are desirable.For instance, in electrical applications, the ability to changeelectrical characteristics of a device to target the device's electricalbehavior to a particular implementation increases the device'sfunctionality and flexibility. Similarly, in chemical sensors, sensing avariety of molecular species using the same sensor or sensor arrangementis advantageous in applications where it is not feasible to use manydifferent sensors (e.g., due to space, cost, response speed or otherlimitations). In previous carbon nanotube implementations, however,achieving high functionality and flexibility has been challenging. Inparticular, it has been difficult to readily remove molecules fromcarbon nanotubes for manipulating properties thereof.

SUMMARY

The present invention is directed to overcoming the above-mentionedchallenges and others related to carbon nanotube devices and theirimplementations. The present invention is exemplified in a number ofimplementations and applications, some of which are summarized below.

According to an example embodiment of the present invention, light isdirected to a carbon nanotube for desorbing molecules therefrom. Inconnection with this example embodiment, it has been discovered thatlight of a selected wavelength and intensity can be used to rapidlydesorb molecules from a carbon nanotube. This rapid photodesorption hasbeen found to be particularly applicable to molecular sensors,nanotube-based molecular electronics and optoelectronic devices.

In another example embodiment of the present invention, a molecularsensor includes a carbon nanotube and a light source adapted to directlight to the carbon nanotube for desorbing molecules therefrom. In thisexample embodiment, the light applied to the carbon nanotube cleans(desorbs) molecules from the carbon nanotube, such that the carbonnanotube can subsequently be used for detecting further molecules viaadsorption. The subsequently adsorbed molecules are detected as afunction of an electrical characteristic of the carbon nanotube, such asconductance, that changes in response to the adsorbed molecules. Thisapproach is particularly useful for rapid recovery with such molecularsensors.

In another example embodiment of the present invention, a circuitarrangement includes a carbon nanotube having undergone photodesorption.In connection with this example embodiment, it has been discovered thatcarbon nanotubes having undergone photodesorption exhibit ambipolarcharacteristics. By applying a gating voltage to the carbon nanotube,p-type, n-type and insulative behavior can each be selectively elicitedfrom the carbon nanotube, providing a flexible circuit.

In still another example embodiment of the present invention, a circuitarrangement includes a plurality of carbon nanotube devices, each of thecarbon nanotube devices having a selectively programmed carbon nanotube.The selective programming is achieved via photodesorption of moleculesfrom one or more of the carbon nanotubes, with the presence and absenceof molecules adsorbed to the carbon nanotube being indicative of twodistinct electrical states (e.g. two conductance states and/or “ON” and“OFF” states).

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and detailed description that follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thedetailed description of various embodiments of the invention thatfollows in connection with the accompanying drawings, in which:

FIG. 1 is a flow diagram for molecular photodesorption from a carbonnanotube, according to an example embodiment of the present invention;

FIG. 2 is a graph showing normalized conductance over time of a carbonnanotube undergoing photodesorption, according to another exampleembodiment of the present invention;

FIG. 3 is a graph showing conductance over time of a carbon nanotubeundergoing photodesorption with ultraviolet (UV) light in a vacuum,according to another example embodiment of the present invention;

FIG. 4 is a graph showing conductance over time of a carbon nanotubeundergoing photodesorption with different intensities of ultraviolet(UV) light, according to another example embodiment of the presentinvention;

FIG. 5 is a graph showing current (I) versus gate voltage (V_(g)) of acarbon device undergoing photodesorption with ultraviolet (UV) light ina vacuum, according to another example embodiment of the presentinvention; and

FIG. 6 is circuit arrangement including a carbon nanotube and a lightsource for molecular photodesorption from the carbon nanotube, accordingto another example embodiment of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

The present invention is believed to be useful for a variety ofdifferent types of devices, and the invention has been found to beparticularly suited for photo-induced desorption with carbon nanotubes.While the present invention is not necessarily limited to suchapplications, various aspects of the invention may be appreciatedthrough a discussion of various examples using this context.

According to an example embodiment of the present invention, it has beendiscovered that light directed to a carbon nanotube induces moleculardesorbtion therefrom, which in turn affects electrical and chemicalproperties of the carbon nanotube. This photo-induced desorption(photodesorption) is useful for removing undesirable molecules from thecarbon nanotube, is highly efficient and desorbs molecules includingboth nearly chemisorbed and physisorbed molecules. For instance, whenusing the carbon nanotube to detect and/or isolate molecules viaadsorption, it is useful to rapidly desorb molecules from the carbonnanotube prior to detecting and/or isolating additional molecules. Inaddition, when using the carbon nanotube in an electric circuit, it isuseful to desorb molecules for manipulating one or more electricalcharacteristics of the carbon nanotube. With this approach, carbonnanotubes can be used to form molecular-scale wires that can be used ina variety of implementations, such as electronic circuits, molecularsensors and optoelectronic devices.

FIG. 1 is a flow diagram for photodesorption from a carbon nanotube,such as a SWNT, according to another example embodiment of the presentinvention. At block 110, molecules are adsorbed to the carbon nanotube.This adsorption may occur, for instance, unintentionally (e.g., when thecarbon nanotube is exposed to air) or selectively by introducing a gasto the carbon nanotube, for example, via a gas inlet to a chamber. Inthe context of such selective adsorption, e.g., the carbon nanotube isexposed to a gas selected for manipulating one or more properties of thecarbon nanotube via adsorption of molecules in the gas. This selectiveadsorption approach is particularly useful, for example, for adsorbingmolecules for selectively setting an electrical characteristic of thenanotube, such as the resistance of the nanotube, for use in amicroelectronic circuit. In another example, this selective adsorptionapproach is useful for making the carbon nanotube selective for furtheradsorption of a second type of molecule for detecting the presencethereof.

At block 120 of FIG. 1, a light source is used to direct light to thecarbon nanotube and to desorb molecules therefrom. An electricalcharacteristic of the carbon nanotube is detected at block 130 and usedas an indication of the quantity and/or composition of the adsorbedmolecules. If the detected electrical characteristic meets a thresholdat block 140, a sufficient amount of the molecules have been desorbedand the light source is stopped at block 150. If the threshold is notmet at block 140, light is again directed at block 120 and theelectrical characteristic detected at block 130 until the threshold ismet.

The threshold used at block 140 is selected to correspond to one or moreof a variety of electrical characteristics, depending upon theparticular application in which the carbon nanotube is to be used. Forexample, when completely (or nearly completely) desorbing molecules fromthe carbon nanotube to remove a molecular species, the threshold isselected to be indicative of the carbon nanotube being free of theadsorbed molecules (e.g., the threshold corresponds to the behavior ofthe carbon nanotube prior to any adsorption). In another example, when aparticular electrical characteristic (e.g., conductance) is desired forimplementation of the carbon nanotube, the threshold is selected tocorrespond to the carbon nanotube exhibiting the desired characteristic.

In another example embodiment of the present invention, characteristicssuch as wavelength and intensity of light directed to the carbonnanotube at block 120 are selected to achieve one or more particularphotodesorption characteristics. In connection with this exampleembodiment, it has been discovered that the photodesorption of moleculesis dependent upon the wavelength of the light. For instance, it has beenfound that ultraviolet light having a wavelength of about 254 nanometersis useful for desorbing Oxygen from a carbon nanotube. As the wavelengthof light varies from 254 nanometers, the desorbtion of Oxygen alsovaries (e.g., the rate of desorption of Oxygen may decrease or evenapproach zero). Also in connection with this example embodiment, it hasbeen discovered that the intensity of the light similarly affects thephotodesorption of the molecules, as discussed in connection with FIG. 4below. In this regard, photodesorption characteristics, such as aparticular rate of desorption, can be controlled by using light ofdifferent wavelengths and/or intensities. This approach is particularlyuseful for controlling the rate of change of characteristics of thecarbon nanotube, such as the conductance of the carbon nanotube, or foroptimizing the wavelength and intensity of light applied to the carbonnanotube for rapid desorption of molecules therefrom, as discussed inconnection with FIGS. 2 and 3 below.

In another implementation, the wavelength of light directed to thecarbon nanotube at block 120 is selected to correspond to the diameterof the carbon nanotube. In connection with this implementation, it hasbeen discovered that photodesorption characteristics for carbonnanotubes vary in relation to the diameter of the carbon nanotube.Therefore, the diameter of the carbon nanotube undergoingphotodesorption is used to select the wavelength of light applied atblock 120. With this approach, it has been discovered that light havinga wavelength of about 254 nanometers is particularly useful forphotodesorption of molecules from a carbon nanotube having a diameter ofabout 1.5 nanometers.

In another particular example embodiment of the present invention, thelight directed at block 120 causes π-electron plasmon excitation in thecarbon nanotube (e.g., the collective excitation of π-electronoscillations in the nanotube), and the plasmon excitation inducesmolecular desorption from the carbon nanotube. More specifically, highelectric fields associated with plasmon excitation enhance hot electrongeneration in the carbon nanotube, and collective electron oscillationsin the carbon nanotube de-excite into single-particle hot electronexcitations. The hot electrons attach to molecules adsorbed to thecarbon nanotube and induce desorption therefrom. Part of the plasmonexcitation energy is dissipated through breaking molecule-carbonnanotube binding.

In another example embodiment of the present invention, FIG. 2 showsconductance (G, being normalized by G_(I)=(2.8 kΩ)⁻¹) versus time (t) inseconds of a SWNT grown using CVD on a catalytically patterned SiO₂/Sisubstrate and controlled contacting with electron-beam lithography.Ultra-violet (UV) light having a wavelength of about 254 nanometers, anintensity of about 2 mW/cm² and a photon flux of about 2.5×10¹⁵/cm²s isdirected at the SWNT at ambient conditions during time intervals 210,220 and 230. The light desorbs molecules (e.g. Oxygen) from the SWNT andthereby lowers the SWNT's conductance. The photodesorption causes areduction of hole-carriers in the SWNT and thus lowers its conductance.

Referring specifically to portion 205 of curve 200, the conductance ofthe SWNT decreases upon illumination with the ultraviolet light.Interval 210 corresponds to the curve portion 205, with the SWNT beingilluminated with the UV light for about 25 seconds. At node 206, the UVillumination is ceased and the conductance of the SWNT recovers(increases) at a rate slower than the rate of decrease, shown by curveportion 207, due to the gradual re-adsorption of molecules (e.g., Oxygenfrom ambient air). With the re-application of UV light at node 208 forinterval 220, the conductance again decreases to node 226 at a levelslightly below the level at node 206, due to the light being applied atan initial conductance lower than that for interval 210. As the UVillumination of interval 220 is ceased, the conductance similarlyrecovers, and the cycle of conductance decrease followed by recovery isagain repeated with interval 220. The approach shown in FIG. 2 may beuseful, for example, for detecting the presence of molecules byadsorption to the carbon nanotube and subsequently cleaning themolecules from the carbon nanotube at the UV illumination intervals 210,220 and 230.

FIG. 3 shows conductance (G, normalized as in FIG. 2) versus time (t) inseconds of a SWNT undergoing photodesorption in a vacuum, according toanother example embodiment of the present invention. A vacuum of about1×10⁻⁸ Torr is drawn on the SWNT, for example, using a vacuum chamber,and UV light having a wavelength of about 254 nanometers is directed atthe SWNT for a time interval 310 of about 500 seconds. The light causesmolecular photodesorption from the SWNT, and the molecules are removedfrom the SWNT via the vacuum. In one particular implementation, inertgas is introduced to the SWNT during the photodesorption. The normalizedconductance of the SWNT, shown with curve 300, continually approacheszero throughout the time interval 310, with no appreciable recoveryafter removal of the light. With this approach, molecules such as Oxygenare readily removed from the SWNT. After the photodesorption, the SWNTcan be subsequently implemented in a variety of circuit arrangements(e.g., by desorbing the molecules from the SWNT, the influence of themolecules upon certain electrical characteristics of the SWNT, such asthe conductance of the SWNT, is removed).

In another example embodiment of the present invention, the intensity ofUV light applied to a carbon nanotube is selected to achieve a desiredconductance for implementation of the carbon nanotube in a particularcircuit arrangement. FIG. 4 shows three specific implementations withdifferent intensities, with curves 410, 420 and 430 each showingnormalized conductance over time, as in the figures above, for UV lighthaving a wavelength, λ, of about 254 nanometers with intensities of 0.02mW/cm², 0.2 mW/cm² and 2.0 mW/cm², respectively. The light intensity maybe varied, for example, using neutral density filters. In curve 410, theUV light intensity maintains the normalized conductance of the carbonnanotube at about 1.00. For a slightly lower conductance, the UV lightintensity is applied as shown in curve 420, with the normalizedconductance approaching about 0.90 at about 900 seconds of illumination.Finally, curve 430 shows UV illumination that achieves a normalizedconductance that approaches about 0.70 at about 900 seconds ofillumination. Each of these three implementations may be used, forexample, for setting the conductance of the carbon nanotubes forimplementation with a particular electronic circuit.

FIG. 5 shows a curve 510 with current (I, in nanoamps) versus gatevoltage (V_(g), in volts) for a circuit including a carbon nanotube overa substrate and a gate adapted to apply a gating voltage (e.g., viacapacitive coupling) to the carbon nanotube, according to anotherparticular example embodiment of the present invention. The carbonnanotube device is first placed in a vacuum chamber, and a vacuum isdrawn on the chamber to about 1×10⁻⁸ Torr. Light is directed to thecarbon nanotube and desorbs molecules (e.g., O₂, NH₃, NO₂ and/or othermolecules) therefrom. This photodesorption may, for instance, be carriedout in a manner similar to the photodesorption discussed in connectionwith FIG. 3.

After the molecules are desorbed, the carbon nanotube device exhibitsproperties similar to an intrinsic semiconductor showing ambipolar FETbehavior (e.g., both n-type and p-type FET behavior). More specifically,the device becomes insulating at V_(g) of between about −8V and +8V,shown by curve portion 530, exhibits electrical transport through thevalence band (e.g., p-type behavior) at curve portion 520 and exhibitselectrical transport through the conduction band (e.g., n-type behavior)at curve portion 540. Therefore, the gate can be used to control thebehavior of the circuit by applying voltages in the ranges shown.

FIG. 6 shows a carbon nanotube circuit arrangement 600 usingphotodesorption for manipulating properties of a carbon nanotube,according to another example embodiment of the present invention. Thecircuit arrangement 600 includes a carbon nanotube 620, such as a SWNT,with opposite ends of the carbon nanotube 620 coupled to two electrodes622 and 624. A light source 630 (e.g., an Ar ion diode laser, He—Nediode laser and/or AlGaAs diode laser) is arranged over the substrate605 for directing light 632 for desorbing molecules from the carbonnanotube 620. In one implementation, the light source 630 is part of thecircuit arrangement 600 and activated, for example, using a controlcircuit also within the circuit arrangement 600. In anotherimplementation, the light source 630 is separate from the circuitarrangement 600 and may be used, for example, during manufacture of thecircuit arrangement 600.

The substrate 605 includes one or more commonly available semiconductorsubstrate materials, such as silicon and silicon-based materials. Theelectrodes 622 and 624 electrically couple the opposite ends of thecarbon nanotube 620 to circuitry 640 in the device via interconnects 642and 644. In one implementation, the electrodes 622 and 624 include acatalyst material, and in another implementation, the catalyst materialis coated with conductive material. For general information regardingcarbon nanotubes, and for specific examples of carbon nanotubesextending from catalyst particles that can be implemented in connectionwith the instant application, reference may be made to theabove-referenced patent documents and publications.

The carbon nanotube 620 operates in the circuit arrangement 600 in amanner similar to that shown in the foregoing figures. Light 632directed at the carbon nanotube 620 desorbs molecules therefrom, and thedesorbed molecules are optionally removed from the circuit arrangement600 (e.g., using a vacuum). The light 632 is controlled to manipulateproperties, such as conductive behavior (including semiconductivebehavior), of the carbon nanotube 620. For example, light can bedirected at the carbon nanotube 620 for altering the conductance of thecarbon nanotube as discussed in connection with FIGS. 2 and 3.

In another implementation, the circuit arrangement 600 further includesa gate 650 in the substrate 605. The gate 650 is arranged for applying agating voltage to the carbon nanotube 620 for controlling the behaviorof the carbon nanotube 620 in a manner similar to that discussed inconnection with FIG. 5. Specifically, the carbon nanotube 620 exhibitsp-type behavior under gating voltages below about −8V, exhibitsinsulative behavior at gating voltages of between about −8V and +8V andexhibits n-type behavior at gating voltages of over about +8V.

In another example embodiment of the present invention, the substrate605 and the carbon nanotube 620 are enclosed in a chamber represented bydashed lines 660, with the chamber being adapted to draw a vacuum on thesubstrate 605. The light source 630 directs the light 632 into thechamber 660 and to the carbon nanotube 620, and a vacuum drawn on thechamber removes molecules desorbed from the carbon nanotube by the light632. Alternatively, inert gas is filled in the chamber 660, which isparticularly useful for preventing further adsorption of molecules tothe carbon nanotube after they have been removed (e.g., due to thevacuum not removing all of the desorbed molecules from the chamber 660).The chamber 660 is then sealed and the light source 630 is removed.After the photodesorption, the carbon nanotube 620 exhibits propertiessimilar to those shown in FIG. 5, and the sealed chamber 660 preventsadditional molecules from contacting and adsorbing to the carbonnanotube 620. The gate 650 is adapted for applying voltages similar tothose shown in FIG. 5, and the carbon nanotube 620 exhibits ambipolarFET behavior.

In one particular implementation, the light source 630 is separate fromthe circuit arrangement 600 and used during a manufacturing process ofthe device 600. Once the molecules are desorbed from the carbon nanotube620, the chamber 660 discussed above is used to prevent furtheradsorption of molecules thereto and the carbon nanotube can beimplemented in a circuit arrangement, where gas molecules are preventedfrom accessing the carbon nanotube 620. In one instance, the chamber 660includes a semiconductor substrate in which the circuit arrangement 600is buried. In another instance, the chamber 660 is a vacuum chamber usedduring manufacture of the circuit arrangement 600, which is subsequentlyburied in a semiconductor substrate that prevents molecules fromadsorbing to the carbon nanotube 620.

In another implementation, the circuit arrangement 600 detects thewavelength of light 632 being applied thereto (e.g., as aphotodetector). As discussed above, the carbon nanotube 620 respondsdifferently to different wavelengths of light. In this regard, thecircuitry 640 is used to detect the conductance of the carbon nanotube620 across the electrodes 622 and 624. As the light 632 is directed tothe carbon nanotube 620, the rate of change in conductance across theelectrodes 622 and 624 is detected and used to identify the wavelengthof the light, as discussed in connection with FIG. 1.

In another implementation, the circuit arrangement 600 is used forsensing molecular species. Molecules are exposed to the carbon nanotube620 and adsorbed thereto. An electrical characteristic of the carbonnanotube 620 is detected via the circuitry 640 and electrodes 622 and624. The electrical characteristic is used to identify the type ofmolecules adsorbed to the carbon nanotube 620, for example, using achange in resistance of the carbon nanotube to detect that a particulartype of molecule has been adsorbed thereto. In one particular instance,the circuit arrangement 600 optionally includes a computer arrangement670 coupled to the circuitry 640 and adapted for comparing theelectrical response of the carbon nanotube 620 to known electricalresponses of a carbon nanotube to a particular molecular species. Whenthe response of the carbon nanotube 620 matches that of a knownresponse, the molecules adsorbed to the carbon nanotube can beidentified. After the molecules have been identified, the light source630 directs light 632 to the carbon nanotube 620 to desorb the moleculestherefrom, and the circuit arrangement 600 is ready for detectinganother molecular species. With this approach, rapid desorption ofmolecules from the sensor is readily achieved, thus making possiblerapid recovery of the sensor circuit arrangement 600 for use indetecting additional molecular species.

In still another implementation, the circuit arrangement 600 isimplemented in a memory arrangement, with the carbon nanotube 620 beingused to store data as a function of molecules adsorbed thereto. Thelight source 630 is configured and arranged to control the data byselectively desorbing molecules from the carbon nanotube 620. In oneinstance, the carbon nanotube 620 is exposed to an environment of aselected gas, such as O₂ and/or NH₃ and/or NO₂, molecules of whichadsorb to the carbon nanotube 620. A first state of the carbon nanotube620 involves the molecules being adsorbed to the carbon nanotube, whichresults in the carbon nanotube 620 exhibiting a first conductance. Asecond state of the carbon nanotube 620 involves the molecules not beingadsorbed to the carbon nanotube (or a different amount of moleculesbeing adsorbed to the carbon nanotube), as controlled by the lightsource 630, which results in the carbon nanotube exhibiting a secondconductance. These first and second states of the carbon nanotube 620are used as first and second memory states, such as “ON” and “OFF”states or “ONE” and “ZERO” states. The presence of the molecules iscontrolled via selective photodesorption from the carbon nanotube 620with the light source 630, which switches the carbon nanotube 620between states. With this approach, data can be written to and erasedfrom the carbon nanotube 620, with the presence or absence of themolecules adsorbed to the carbon nanotube 620 effectively being the“data.” The circuitry 640 is thus used to detect the state that thecarbon nanotube 620 is in and to read out the state, such as for readingout a “ONE” or a “ZERO.”

With the memory-arrangement approach discussed above in connection withFIG. 6, the carbon nanotube 620 may be implemented in a variety ofmemory applications. In one instance, the carbon nanotube 620 isreplicated in an array of nanotube-memory cells (not shown) havingmolecules adsorbed thereto for read-only memory, with selective ones ofthe nanotube-memory cells being erased via photodesorption during amanufacturing process for the replicated carbon nanotubes.Alternatively, molecules are selectively adsorbed to the replicatedcarbon nanotubes, either in connection with or independently from thephotodesorption.

In another particular implementation, photodesorption from carbonnanotubes is used to prevent reverse engineering of a circuitarrangement, such as the memory arrangement discussed above inconnection with FIG. 6. For example, a carbon nanotube is erased in avacuum and sealed in a circuit arrangement. When the nanotube is exposedto for reverse engineering, molecules such as Oxygen in air desorb ontothe carbon nanotube, permanently altering the electrical characteristicsof the circuit arrangement including the carbon nanotubes. Therefore,the state of conductance of the carbon nanotubes prior to exposurecannot necessarily be detected.

In another particular implementation, an array of carbon nanotubes isprogrammed by selectively adsorbing molecules to individual ones of thecarbon nanotubes using, for example, the adsorption and photodesorptiontechniques discussed above. To deprogram the array, the carbon nanotubesare exposed to light to erase data (molecules) stored on the carbonnanotubes. This approach is particularly useful, for example, forone-time usage of data programmed onto the carbon nanotubes, as with aconventional read-only memory, such as EPROM.

In another instance, the circuit arrangement 600 is used as a fusiblelink for enabling or disabling an electrical connection, such as forenabling or disabling memory cells. By controlling the conductance ofthe carbon nanotube 620 via photodesorption of molecules adsorbedthereto, the carbon nanotube 620 can be used as a link, or switch, toclose a circuit across the nodes 620 and 622. This fusible link isparticularly useful for enabling or disabling circuitry, such as aparticular memory bank or a particular circuit element. In addition, bymaking access to the carbon nanotube 620 difficult (e.g., by burying thecarbon nanotube 620 in a semiconductor substrate), reverse engineeringof the circuit in which the carbon nanotube is implemented is impededdue the difficulty in detecting the conductance state of the carbonnanotube. For general information regarding fusible links, and forparticular information regarding fusible links to which the presentinvention may be applied, reference may be made to U.S. Pat. No.5,532,966 (Poteet, et al.), which is fully incorporated herein byreference.

The various embodiments described above are provided by way ofillustration only and should not be construed to limit the invention.Based on the above discussion and illustrations, those skilled in theart will readily recognize that various modifications and changes may bemade to the present invention without strictly following the exemplaryembodiments and applications illustrated and described herein. Forinstance, such changes may include modifying the carbon nanotubes forsensing one or more particular molecular species, altering the circuitarrangements, and where appropriate, using the SWNTs as building blocksfor more complex devices. Such modifications and changes do not departfrom the true spirit and scope of the present invention. In addition,for general information regarding carbon nanotubes, and for specificinformation regarding carbon nanotube implementations that may be usedin connection with the present invention, reference may be made toAppendices A and B, which form part of the underlying patent documentand are fully incorporated herein by reference.

1. A method for using a carbon nanotube device including a carbonnanotube, the method comprising: using a light source to direct light atthe carbon nanotube and to desorb molecules from the carbon nanotube;detecting a change in an electrical characteristic of the carbonnanotube; and using the detected change to detect that the moleculeshave been desorbed from the carbon nanotube.
 2. The method of claim 1,wherein using a light source to direct light at the carbon nanotubeincludes exciting plasmons in the carbon nanotube with the directedlight.
 3. The method of claim 1, wherein using a light source to directlight at the carbon nanotube includes directing ultraviolet (UV) lightat the carbon nanotube, the UV light having a wavelength that issufficient to desorb the molecules from the carbon nanotube.
 4. Themethod of claim 3, wherein directing UV light at the carbon nanotubeincludes directing UV light having a wavelength of about 254 nanometers.5. The method of claim 4, wherein directing UV light at the carbonnanotube includes directing UV light having an intensity of about 2.0mW/cm² at the carbon nanotube.
 6. The method of claim 1, furthercomprising drawing a vacuum on the carbon nanotube and preventingmolecules from adsorbing to the carbon nanotube, after using a lightsource to desorb molecules from the carbon nanotube.
 7. The method ofclaim 1, further comprising: applying a voltage across the carbonnanotube before the step of detecting a change in an electricalcharacteristic of the carbon nanotube.
 8. The method of claim 1, priorto using a light source to direct light at the carbon nanotube, furthercomprising: introducing molecules to the carbon nanotube; detecting anelectrical characteristic of the carbon nanotube and detecting thepresence of the molecules via the detected electrical characteristic;and wherein desorbing molecules from the carbon nanotube includes usingthe light source to purge the introduced molecules from the carbonnanotube.
 9. The method of claim 1, further comprising sealing thecarbon nanotube in a circuit arrangement that prevents molecules fromadsorbing to the carbon nanotube, after using a light source to desorbmolecules from the carbon nanotube.
 10. A method for using a carbonnanotube device including a carbon nanotube, the method comprising:using a light source to direct light at the carbon nanotube and todesorb molecules from the carbon nanotube; drawing a vacuum on thecarbon nanotube and preventing molecules from adsorbing to the carbonnanotube, after using a light source to desorb molecules from the carbonnanotube; and applying a gating voltage to the carbon nanotube.
 11. Themethod of claim 10, wherein applying a gating voltage to the carbonnanotube includes applying the gating voltage such that the carbonnanotube exhibits electrical transport through the valence band inresponse to applying a high negative voltage thereto, exhibitsinsulative behavior in response to applying about zero voltage theretoand exhibits electrical transport through the conduction band inresponse to applying a high positive voltage thereto.
 12. A molecularsensor comprising: a carbon nanotube; means for introducing molecules tothe carbon nanotube, the molecules adsorbing to the carbon nanotube andchanging an electrical characteristic thereof; a detection circuitconfigured and arranged to detect the changed electrical characteristicof the carbon nanotube and to detect the presence of the adsorbedmolecules via the changed electrical characteristic; and a light sourceconfigured and arranged to direct light to the carbon nanotube, afterdetecting the presence of the adsorbed molecules, and to desorb themolecules from the carbon nanotube.
 13. The molecular sensor of claim12, wherein the detection circuit is further configured and arranged fordetecting the composition of the molecules adsorbed to the carbonnanotube via the detected changed electrical characteristic.
 14. Acircuit arrangement comprising: a carbon nanotube; circuitry coupledacross the carbon nanotube; and a light source configured and arrangedto direct light to the carbon nanotube for desorbing moleculestherefrom.
 15. The circuit arrangement of claim 14, wherein the lightsource is adapted to change the conductance of the carbon nanotube viathe directed light.
 16. The circuit arrangement of claim 14, furthercomprising: a chamber that includes the carbon nanotube; and a vacuumarrangement configured and arranged to draw a vacuum on the chamber. 17.The circuit arrangement of claim 16, wherein the vacuum arrangement isconfigured and arranged to remove desorbed molecules from the chamber.18. The circuit arrangement of claim 14, further comprising a siliconsubstrate having a gate therein, the carbon nanotube being over the gatein the silicon substrate, the gate being configured and arranged tocapacitively couple a voltage to the carbon nanotube.
 19. The circuitarrangement of claim 18, wherein the carbon nanotube exhibits electricaltransport through the valence band in response to the gate capacitivelycoupling a high negative voltage thereto, exhibits insulative behaviorin response to the gate capacitively coupling about zero voltagethereto, and exhibits electrical transport through the conduction bandin response to the gate capacitively coupling a high positive voltagethereto.
 20. The circuit arrangement of claim 14, wherein the lightsource is configured and arranged to apply ultraviolet light to thecarbon nanotube.
 21. The circuit arrangement of claim 14, wherein thelight source is configured and arranged to direct light having awavelength of about 254 nanometers.
 22. The circuit arrangement of claim14, wherein the carbon nanotube exhibits a conductance that is afunction of the wavelength of the light being directed thereto, andwherein the light source is configured and arranged to control theconductance of the carbon nanotube via the wavelength of the lightdirected thereto.
 23. The circuit arrangement of claim 14, furthercomprising a gas supply configured and arranged to introduce a gas thatattaches to the carbon nanotube and to increase the conductance of thecarbon nanotube via the attached gas.
 24. The circuit arrangement ofclaim 14, wherein the carbon nanotube exhibits a conductance that is afunction of the intensity of the light being directed thereto, andwherein the light source is configured and arranged to control theconductance of the carbon nanotube via the intensity of the light.
 25. Amemory arrangement comprising: a carbon nanotube; a source configuredand arranged to introduce molecules to the carbon nanotube for adsorbingthe molecules to the carbon nanotube; a light source configured andarranged to direct light at the carbon nanotube and to selectivelydesorb said molecules from the carbon nanotube; memory circuitryelectrically coupled to the carbon nanotube and configured and arrangedfor detecting an electrical characteristic of the carbon nanotube, theelectrical characteristic being responsive to the molecules; and whereinthe detected electrical characteristic exhibits a first state inresponse to the carbon nanotube having the molecules adsorbed theretoand wherein the detected electrical characteristic exhibits a secondstate in response to the carbon nanotube not having the moleculesadsorbed thereto.
 26. The memory arrangement of claim 25, wherein thememory circuitry is configured and arranged for reading out a firstvalue in response to the detected electrical characteristic exhibitingthe first state and for reading out a second value in response to thedetected electrical characteristic exhibiting the second state.
 27. Thememory arrangement of claim 25, wherein the light source is configuredand arranged to selectively desorb the molecules in response to a writeaccess to the carbon nanotube.
 28. The memory arrangement of claim 27,wherein the light source is configured and arranged to switch the carbonnanotube between the first and second states, via the directed light, inresponse to signals applied to the light source.